Printing is basically a binary system where images and text are reproduced by the presence or not of ink. In order to reproduce areas with color densities between the full color and the no color case, what is normally referred to as a grey scale, a printing system has been developed, wherein different printing densities are reproduced using a plurality of closely spaced printing dots, arrayed in patterns, sizes and densities such that when viewed by an observer, they appear to have a desired density between “white” (no ink) and “black” (fully inked). Printing by such a process is known as “halftone” printing. It should be pointed out at the onset that use of the terms black, white and grey do not refer to actual black colors, but to different levels of optical density going from absence of printed ink to fully covered with ink, whatever the ink color may be.
There are two basic categories of halftone screen systems: Area Modulation (AM) and Frequency Modulation (FM). With AM halftone screens, dots are equally spaced on a grid and different grey levels are produced by varying, or “modulating” the area (size) of the halftone dot. With FM halftone screens, dots are the same size and different grey levels are produced by varying the frequency (or number) of halftone dots. In practice, dots in FM screens are randomized in such a way that the results may not appear as a “pure” frequency modulated screen. Such screens are also known as “stochastic” halftone screens, but will be referred to as FM screens herein.
In both AM and FM systems, the eye integrates the plurality of “black” dots with “white” paper and perceives a continuous apparent grey level proportional to the ratio of ink to paper area. Such tonal areas are commonly referred to as “dot percentage” or “dot area” and are quantified on a scale of 0% to 100% where 0% is pure “white”, 100% is pure “black” and 50% is a “medium” shade of grey. Both AM and FM screens are also characterized by their physical resolution. With AM screening, physical resolution is expressed as the number of grid lines (or dots) in a linear unit of distance. This measure is known as the “screen ruling”. A typical value might be 100 lines per inch for lower quality newspapers or 200 lines per inch for high quality commercial brochures. With FM, the physical resolution is expressed as by dot size. With FM screens, the size of the dots, and therefore the number of dots in a unit area, governs the resolution of the screen. A typical value might be 50 microns for lower quality newspapers or 20 microns for high quality commercial brochures.
Printing of an image typically involves creating a printing plate which contains both solid image areas and halftone areas, mounting the plate onto a printing press, applying ink onto the solid and halftone areas of the plate surface and transferring the inked image onto a receiving surface. The receiving surface is typically paper, but may also be film, foil, or a nonwoven or other material. Such surfaces are commonly referred to as the substrate. In one of the most common printing techniques used today, offset lithography, the printing plate imaged areas are hydrophobic while the non-imaged areas are hydrophilic. Ink and water are applied to the plate and the ink and water form films over the hydrophobic and hydrophilic areas respectively. The image is then transferred, or “offset” onto an intermediate “blanket” cylinder, from which it is transferred to the substrate.
Excessive ink film thickness on a halftone dot may have numerous negative implications, including loss of color saturation, a loss of print stability, and a propensity to form a mottled appearance. It is therefore desirable to control, in some way, ink film thickness. Printing presses possess the ability to regulate the volume of ink applied to the plate. However, they cannot discriminate between solid image areas, which require large volumes if ink to achieve the desired ink film thickness and level of darkness, and halftone dots, which perform better with thinner ink films. Dots in the range of about 0% to 90% typically perform optimally with lower ink film thickness, while dots above 90% typically require higher ink film thickness to achieve the desired level of darkness. Finally, it should be noted that the change in ink loading in going from low to high percent density is preferably smooth and gradual, so that the change in darkness in the final image appears continuous to the human eye.
A number of factors affect the ink transfer from the plate to the receiving surface, including pressure between these surfaces and the amount of ink present on the ink-receptive areas of the plate. Control of ink film thickness may be difficult, due to these and perhaps other parameters, and the resulting variations may result in a variety of problems. For example, it has been observed that traditional AM halftone screens may suffer from less than ideal stability on press and less than ideal color saturation, due to the relatively thick ink film on the halftone dots of the printing plate. FM screens tend to have greater stability on press and color saturation since the smaller dots tend to carry thinner films of ink. However, although FM printing often provides increased stability on press and color saturation, it tends to produce images that are somewhat “grainy” in appearance when compared with AM printed images, due to the inevitable clumping of the randomly placed dots. It would therefore be desirable to provide a halftone screens system that combines the generally good stability on press and color saturation of FM printing with the absence of graininess achieved by AM halftones.